Frame-Relay Question

jayhuddleston · ‎09-17-2010

I am studying for the CCNA and I came across this question

in the CCNA ICND2 Second Edition of the Wendell Odom book.

I am thinking that the answer to the question would be A and C because it would use the local DLCI. the answer is at the bottom. Can someone tell me if the question is incorrect or explaine why it would use a non-local DLCI. (Why would it use answer B).

Question:

A Frame Relay planning diagram shows the number 101 beside R1, 102 by R2, 103 by R3, and 104 by R4. No other DLCIs are listed. The lead network engineer tells you that the planning diagram uses global DLCI addressing and that a full mesh of VCs exists. Which of the following are true?

a. Frames sent by R1 to R2, as they cross R2’s access link, have DLCI 102.

b. Frames sent by R1 to R2, as they cross R2’s access link, have DLCI 101.

c. Frames sent by R3 to R2, as they cross R3’s access link, have DLCI 102.

d. Frames sent by R3 to R1, as they cross R3’s access link, have DLCI 102.

Answer:

5.

B and C. The global DLCIs represent the DLCI the other routers use when sending frames over their local access links. So, when R1 sends a frame to R2, when the frame crosses R2’s access link, the network has changed the DLCI to R1’s global DLCI, 101. Similarly, when R3 sends a frame to R1, as the frame crosses R3’s access link, the frame has R1’s global DLCI in it, 101.

Thank you in advance.

Peter Paluch · ‎09-17-2010

Hello Jay,

The global DLCI addressing is somewhat of a peculiarity. The basic idea is that a single router can be reached under the same DLCI number from all other member routers in the FR network. In your case:

The router 1 can be reached under DLCI 101 from routers 2, 3, and 4
The router 2 can be reached under DLCI 102 from routers 1, 3, and 4
The router 3 can be reached under DLCI 103 from routers 1, 2, and 4
The router 4 can be reached under DLCI 104 from routers 1, 2, and 3

Let's focus, for example, on routers 1, 2, and 3 communicating with the router 4. If routers 1, 2 or 3 are sending a frame to router 4, they will all use the DLCI 104 to send their frames. However, as the individual frames emerge on the access link to router 4, the DLCI must be rewritten to uniquely identify the sender of the frame. Thus, the frame from router 1 will arrive on router 4 with the DLCI 101, the frame from router 2 will arrive on router 4 with the DLCI 102, and the frame from router 3 will arrive on router 4 with the DLCI 103. Why? Because it is precisely the DLCI 101, 102, and 103 that is used by the router 4 when it sends replies to the routers 1, 2 and 3.

Following this logic, the correct answers are indeed B and C. Use this simple shortcut:

When sending the frame, the originating router uses the DLCI of the destination to send the frame
When receiving the frame, the frame will be received on the DLCI that can be used to reach the sender of the frame

I hope this helps to clear this mystery a bit but please ask further if there is anything unclear.

Best regards,

Peter